1. Field of the Invention
The present invention relates to monitoring the breathing of medical patients, and more particularly to devices and methods for indicating instantaneously the tidal volume of each patient breath.
2. Description of the Prior Art
Efficient utilization of the typical respiratory system ventilator is frequently impaired by the lack of immediate information regarding the tidal volume delivered with each inhalation cycle. It is a common practice in ventilator design to direct a regulated constant flow of gas past a patient interface and out to ambient air through an exhalation valve. When the patient inhales by his own effort he merely diverts part of the gas flow into his lungs during his inhalation cycle. When it is desired that the ventilator supply the inhalation energy, the exhalation valve is programmed to open and close intermittently to force inhalation whenever the exhalation valve is closed.
If the flow rate and the time of exhalation valve closure are known, the force-inhalation tidal volume can be calculated. This tidal volume calculation requires an accurate flow measuring and indicating device, an accurate time measuring and indicating device and an accurate calculating capability. Even if the accurate flow and time measuring and indicating devices are provided, the calculation must be performed any time the flow rate or inhale cycle time is changed, and the net result in practice is to ignore the tidal volume parameter or to avoid making a flow and/or time change that might optimize the ventilator-patient interchange.
The need for an accurate and sensitive tidaly volume indicator has thus existed for some time, and attempts to provide one are illustrated in U.S. Pat. Nos. 3,789,837 and 3,898,987. A review of the disclosure in these patents however, illustrates the complicated arrangements previously arrived at. For example, in U.S. Pat. No. 3,898,987 a bag is expanded and collapsed in timed relation to the respiration of the patient, which in turn moves a ball up and down in a tube to indicate the tidal volume of respiration. This arrangement requires that a plurality of interrelated parts function with each other to obtain the desired end results.
In a similar manner, a review of the disclosure in U.S. Pat. No. 3,789,837 will similarly illustrate a complicated machanical arrangement intended to provide a visual display of tidal volume. In contrast to the above disclosures, I have discovered a substantially simplified system for obtaining a visual readout of tidal volume that additionally has various other advantages over the prior art which will become apparent as the disclosure proceeds.